Tuesday, October 19, 2010

Mating isolation is a frequent contributor to ecological speciation – but how consistently does it evolve as a result of divergent selection? An excellent model system to address this question is the threespine stickleback, with its species pairs that evolved multiple times in a diversity of contrasting habitats. Mating isolation could arise through assortative mate choice, such as when females actively choose a male of their own type or when male-male competition does the job for them.

We designed an experiment aiming to demonstrate that at least one of these possibilities would lead to mating isolation in­ the Misty watershed on Vancouver Island, British Columbia. This watershed contains a very divergent but interconnected pair of lake-inlet stream stickleback. In the laboratory, gravid females were introduced into a tank where they could choose between, and be competed for, by two males of different types: lake, stream, or hybrid. Despite very different courtship behaviours of lake and stream males, and seemingly inferior levels of competition of hybrid males, mating patterns were not assortative. In fact, hybrid males obtained the most females. It is hard to define stickleback attractiveness if you are not a stickleback yourself, but hybrid males were reasonably large and aggressive (unlike stream males) and showed some elegance in their courtship (unlike lake males). This combination may have promoted their high mating success.

These results stand in contrast to other stickleback pairs which do show assortative mating, such as the benthic-limnetic and anadromous-freshwater pairs. So far, other studies in the Misty system also haven't found strong reproductive barriers. This is either because we haven't been looking close enough to find the real ecological cause(s) of reproductive isolation, because the real cause(s) are not visible in standard laboratory experiments, or because there is no strong progress towards ecological speciation. Either way, our results highlight the importance of considering the various factors that promote or constrain progress toward ecological speciation.

And just when we were checking the online version of our paper, a very similar study appeared online. Christophe Eizaguirre and colleagues from the Max-Planck Institute for Evolutionary Biology (Plön, Germany) showed that in their lake and stream system, stickleback do choose assortatively based on olfactory cues in a flow channel. Previous studies at MPI Plön have shown that lake and stream stickleback are infected by different parasite communities, and are highly divergent for MHC, a set of immune genes. Interestingly, these genes are also involved in mate choice. Being under both natural and sexual selection, MHC has the potential to act as a “magic trait”, accelerating ecological speciation.

The standing contrast then is that Plön stickleback choose their own type (lake or stream) based on olfactory cues, whereas Misty stickleback don't choose their own type (lake or stream) based on olfactory cues and visible cues together. The latter situation is certainly a more realistic setting. However, another contrast between our experiments was that Misty stickleback were second generation lab-reared fish, while Plön stickleback were captured in the wild. This is important as mating preferences could be learned based on imprinting on conspecifics or experience with heterospecifics (Kozak et al. 2009; Behav. Ecol). Much more work needs to be done to assess the consistency of mating barriers in stickleback, perhaps through reciprocal experiments: what if Misty sticklebacks can only smell their potential mates, and what if Plön sticklebacks can actually see their potential mates? To be continued...